The prior art provides various tool steels for use in producing items such as tools and dies. Such tool steels are generally classified as: (i) relatively low-alloy tool steels having higher hardenability than plain carbon steels; (ii) intermediate alloy steels which usually contain elements such as tungsten, molybdenum or vanadium, which form hard, wear resisting carbides; and (iii) high-speed tool steels containing large amounts of carbide forming=elements which serve not only to furnish wear resisting carbides but also to promote secondary hardening and thereby increase resistance to softening at elevated temperatures.
The relatively low-alloy and intermediate tool steels are commonly employed to produce dies which are utilized to shape, form, bend, draw, cut or otherwise process low carbon steels, stainless steels and aluminum. Such materials prior to processing may assume any one of a variety of configurations such as, for example, bars, rods, strips or sheets. The automotive industry, which does a considerable amount of metal processing, utilizes various low-alloy and intermediate alloy tool steels to produce dies. Such dies are commonly employed in presses and are used to produce items such as, for example, hoods, fenders, roof decks and trunk lids. The automotive industry places some fairly critical demands upon the tool steels from which their dies are produced. More particularly, many automotive dies undergo a considerable amount of machining and grinding in order to allow the die to produce items of intricate shape and exacting size tolerances. Also, automotive dies are many times used to process a tremendous number of items and are thus subject to very long runs. Additionally, some automotive dies are very large in size and require a considerable amount of tool steel in their production. Thus, preferably the tool steel does not include major amounts of expensive alloying elements because the cost of the tool steel itself can be a significant factor in the construction of the dies.
An example of one tool steel utilized by the automotive industry to produce dies is a tool steel sold by the Uddeholm Corporation of Sterling Heights, Mich., under the trademark FERMO. Generally, FERMO tool steel would be classified as a relatively low-alloy tool steel having about 0.45 to 0.52 percent by weight carbon, 0.75 to 1.05 percent by weight manganese, 0.40 to 0.80 percent by weight silicon and 1.30 to 1.70 percent by weight chromium. FERMO tool steel is preferred by some automotive personnel because it tends not to distort during flame hardening. Also, FERMO tool steel may be welded cold thereby facilitating repair of the die while it is mounted in the press or similar machine. Thus, such dies do not have to be removed from the press thereby helping to minimize costly downtime. Unfortunately, FERMO tool steels generally display a maximum Rockwell (R) hardness on the C-scale (Rc) of about 54 to 58. Thus, FERMO tool steel is generally not suited for long runs where a die is scheduled to be utilized to produce a great number of items or pieces.
An example of another tool steel utilized by the automotive industry includes about 0.85 to 1.0 percent by weight carbon, .20 to 0.30 percent manganese, 0.20 to 0.30 percent by weight silicon and 0.15 to 0.25 percent by weight vanadium. This tool steel is preferred by some automotive personnel because it can be repair welded in the press. However, flame hardening of this tool steel is conducted at a temperature of about 1600.degree. F. to 1650.degree. F. followed by a water quench. Unfortunately, distortion has been found many times to develop during this hardening treatment.
An example of another tool steel utilized by the automotive industry includes about 0.85 to 1.10 percent by weight carbon, 0.50 to 0.70 percent by weight manganese, 0.25 to 0.40 percent by weight silicon, 4.75 to 5.25 percent by weight chromium, 0.20 to 0.40 percent by weight vanadium and 0.95 to 1.20 percent by weight molybdenum. This tool steel is commonly flame hardened at a temperature of about 1800.degree. F. or higher and generally displays a Rockwell hardness on the C-scale of around 60. This tool steel usually cannot be repair welded while the die is in the press. Generally, the damaged die, or sections thereof, must be removed from the press and preheated prior to repair welding. This can be a time-consuming process leading to costly downtime.
Another tool steel utilized in the automotive industry includes about 0.45 to 0.55 percent by weight carbon, 1.0 to 1.20 percent by weight manganese, 0.30 to 0.50 percent by weight silicon, 1.00 to 1.25 percent by weight chromium and 0.35 to 0.45 percent by weight molybdenum. This alloy is generally supplied in an annealed condition having a Brinell hardness number (BHN) of about 180 to 220. Flame hardening of this tool steel is generally conducted at a temperature of about 1780.degree. F. followed by a water quench to produce a Rockwell hardness on the C-scale of about 58 to 60. Problems experienced by some automotive personnel with this tool steel include distortion during flame hardening and a relatively low wear resistance.
Generally, the aforementioned steels are formed into dies while the tool steel is in an annealed and/or normalized condition. In order to attain this condition, such tool steels are generally annealed and/or subjected to a normalization treatment until the desired hardness is attained.
Relatively low alloy tool steels are also used in some applications to produce tools such as chisels. An example of a tool steel that was at one time utilized to produce chisels contained about 0.35 percent by weight carbon, 0.70 percent by weight manganese, 0.45 percent by weight silicon, 0.80 percent by weight chromium, 0.30 percent by weight molybdenum and 0.30 percent by weight copper. This tool steel was preferred for such applications as chisels because of its tendency not to become brittle and break during use. This tool steel generally would not be used to produce dies because of its inability to consistently produce Rockwell hardnesses on the C-scale in excess of about 54.
A recently developed flame hardenable tool steel and methods of heat treating such steel are disclosed in U.S. Pat. Nos. 5,182,079 and 5,055,253. These patents disclose a tool steel comprising by weight 0.50-0.65 percent carbon, 0.090-1.45 percent manganese, up to about 0.030 percent phosphorus, 0.035-0.070 percent sulfur, 1.10-1.90 percent chromium, 0.15-0.40 percent nickel and 0.20-0.40 percent copper. The '079 patent discloses two different heat treat schedules. One schedule concerns castings over 120 pounds and the other schedule concerns castings below 120 pounds. Generally, for castings below 120 pounds the tool steel is preheated, austenitized and double tempered. For castings above 120 pounds the tool steel is normalized followed by air-cooling.